This application is based on and claims the priority date of German Application No. 101 41 431.5, filed on Aug. 23, 2001, which is incorporated herein by reference.
To avoid load changing losses with a high cylinder load, the gas exhaust valves on piston internal combustion engines must open prior to the end of the expansion cycle, at a point in time when the pressure inside the cylinder still registers values of several bars. The valve actuation force in that case not only must overcome the valve spring counter-acting force and the inertial force, but also the gas pressure force on the cylinder interior that acts upon the valve disk or poppet assembly of the gas exhaust valve.
With gas cylinder valves actuated by camshafts, the additionally required force is generated without problem via the camshaft drive.
However, with piston internal combustion engines provided with gas cylinder valves that are actuated with the aid of fully variably controlled actuators, in particular electromagnetic actuators, the relatively high cylinder interior pressure still present at the start of the opening has a negative effect. This effect is noticed in the higher actuation force that must be generated to open the exhaust valve. The higher force requirement can be met, for example, by providing a stiffer valve opening spring in the actuator, so that the valve opening spring is correspondingly pre-tensioned during the valve closing with the aid of a higher magnetic force and thus a higher electrical energy at the closing magnet.
It is the object of the invention to support the opening of a gas exhaust valve by influencing the flow conditions in the gas exhaust.
This object is achieved according to the invention with a piston internal combustion engine having at least one gas exhaust valve for each cylinder. The gas exhaust valve is actuated by an actuator, in particular an electromagnetic actuator, which is controlled fully variable by an engine control and can close off the cylinder interior space in accordance with the predetermined operating cycle against a gas exhaust channel that adjoins a valve seat and is connected to an exhaust gas system. The exhaust gas system is provided with means for reducing the pressure gradient between the cylinder interior space and the following gas outlet channel at the start of the gas exhaust valve opening. The advantage of this type of arrangement is that pressure at the constriction or bottleneck already exists at the start of the opening, that is with a small opening cross section at the valve seat, as a result of a specific, periodic pressure build-up in the gas exhaust channel. This pressure is higher than the normal, low counter pressure in the exhaust gas system. The gas force to be overcome by the actuator, which acts from the cylinder interior space upon the gas exhaust valve, is thus reduced short-term at a time when the valve opening spring is still tensioned almost completely. The energy stored in the valve opening spring is therefore sufficient for transferring the valve to the location for capturing the armature of the opening magnet. Subsequently, the gas exhaust valve can be opened fully and practically without increased energy expenditure.
For one embodiment of the invention, the constriction functions as a means for reducing the pressure gradient in the gas exhaust channel.
One advantageous embodiment of the invention provides that the constriction is arranged near the valve seat. The volume delimited by the valve seat on the one hand and the constriction on the other hand can thus be kept as small as possible. The desired pressure therefore builds up quickly and the pressure difference between the pressure on the inside of the cylinder and the pressure in the adjacent exhaust gas system is reduced correspondingly quickly.
Another advantageous embodiment of the invention provides that in the region following the constriction in a flow direction of the exhaust gases, the open flow cross section expands in the manner of a diffuser, at least over a partial length of the gas exhaust channel. As a result, the exhaust gas that is pushed by the piston out of the cylinder chamber can flow off quickly and without problems during the course of the further opening of the gas exhaust valve.
With an arrangement of two gas exhaust valves for each cylinder, it is particularly advantageous if the two gas outlet or exhaust channels converge into a single channel, wherein the constriction is arranged in the area of the convergence.
According to a different, advantageous embodiment provided with two gas exhaust valves for each cylinder, one gas exhaust valve is designed to have a constriction and the other gas exhaust valve is designed without a constriction. With a corresponding layout of the engine control in predetermined engine-lcad ranges and utilizing the above-described reduction in the pressure difference, this arrangement allows the gas exhaust valve at the gas exhaust channel provided with a constriction to open ahead of the other gas exhaust valve, particularly in the range of high engine loads. This ensures that the actuator for the other gas exhaust valve, to which a xe2x80x9cnormalxe2x80x9d gas exhaust channel is assigned, practically does not have to overcome any gas force because the high gas force inside the cylinder chamber is already reduced via the gas exhaust valve relieved of pressure due to the build-up of a counter-pressure during the opening operation.